The present invention relates to an apparatus for controlling a steering force of a hande in an automobile, and more specifically to an apparatus for controlling a steering force of a handle for controlling operation of a power steering system in response to the speed or steering angle of the automobile.
It is widely known, for example, from U.S. Pat. Nos. 3,433,127, 4,034,825, 4,119,172 and the like that rotational torque from a steering handle is transmitted to the travelling wheels of a vehicle after the torque has been increased by a power steering system provided with an elastic torsional bar. Particularly, the latter U.S. Pat. No. 4,034,825 discloses that when the speed of the automobile is high, the function of the power steering system is decreased whereas when the speed is low, the function is increased.
In such a power steering system, rotation of a rotary shaft of the engine, for example, is transmitted to an oil pump by a pulley and an endless belt extended over the pulley, and oil in an oil tank is supplied to the power steering system by the oil pump to assist the steering force. Further, rotation of the counter shaft of the transmission of the engine is transmitted to an auxiliary oil pump, and oil from the oil tank is suctioned by the auxiliary oil pump. A throttle valve is provided on an outlet of the auxiliary oil pump and, oil having passed through the throttle valve is again returned to the oil tank and pressurized oil is introduced from between the outlet of the auxiliary oil pump and the throttle valve to a hydraulic reaction chamber for controlling the torsion of the torsional bar to control the function of the power steering system. This is, the auxiliary pump increases the number of revolutions thereof proportional to the vehicle speed by the counter shaft of the transmission to increase the quantity of discharge. Accordingly, at the time of high speed, high oil pressure is applied to the throttle valve, as a consequence of which high pressure is applied to the hydraulic reaction chamber to weaken the function of the power steering system thereby making the handle operation heavy. However, there is a drawback at low speed rotation of that in the counter shaft, the quantity of discharge of the auxiliary pump is small to failing to obtain a great hydraulic reaction. There is a further drawback in that since the counter shaft is positioned away from the oil tank or hydraulic reaction chamber, a large space is required for housing the piping. When the vehicle speed is low, the quantity of discharge from the auxiliary pump is also insufficient whereby pulsation and fluctuation in pressure occur.
As shown in FIG. 3, A, characteristics of torque/pressure of the power steering system is such that at the time of high speed, the steering force becomes excessively small, and it is difficult to maintain the straight-ahead condition of the automobile and the steering handle is excessively operated often imparting uneasiness to the vehicle operator. Therefore, the hydraulic reaction chamber as described above is provided, and the reaction pressure is controlled by the vehicle speed to thereby widen a senseless zone of the hydraulic reaction chamber as shown in FIG. 3, B and C. However, as will be understood from the characteristics of B and C, since the rise in pressure relative to the increase in torque after the passage of the senseless zone is acute, the steering force is suddenly small when the steering torque exceeds the senseless zone such as in cases where rapid handle operation is effected or the steering angle is great, and the handle is excessively operated, which states may not be improved. Where control is made merely by vehicle speed, in the characteristics of torque/pressure, a senseless zone portion is merely widened, and the contour of the characteristic curve beyond the sensless zone is the same, sometimes lacking in a handle-operating feeling.